1. Field of the Invention
The present invention relates to a ceramic composite which has a high mechanical strength, and an excellent creep resistance in a wide temperature range from room temperature to a high temperature and is suitable for use as a structural material at a high temperature.
2. Description of the Related Art
SiC and Si.sub.3 N.sub.4 have been investigated to develop ceramic materials to be used at high temperatures but they are not sufficient in high temperature properties. As an alternative material thereof, SiC/SiC composite materials produced by chemical vapor impregnation, provided by Societe Europeene de Propulsion, have attracted attention, and at the present are considered to be the best high temperature structural materials that have have been investigated and developed. The temperature range at which they can be used is reported to be 1400.degree. C. or lower.
Manufacture of ceramics is primarily done by a powder sintering method in which improvement in powder characteristics, such as finer particle size and higher purity, have made it possible to obtain ZrO.sub.2 ceramics having the high strength, at room temperature, of 30 GPa. It is also possible to produce a composite material in which additional ceramic particles are dispersed at the level of nano meters in a sintered ceramic material, by which an improvement in the strength, toughness and thermal properties of the ceramic material was provided.
It was generally considered that oxide ceramics cannot be used for high temperature structural materials which receive high loads, since oxide ceramics are easily deformed at high temperatures. The oxide ceramics are better in resistance to oxidation and corrosion at high temperatures than other types of ceramics and therefore can be expected to be used in a wide range of uses if the mechanical strength at high temperature can be enhanced. In this respect, metal oxides such as Al.sub.2 O.sub.3, ZrO.sub.2 and MgO and rare earth element oxides such as Y.sub.2 O.sub.3, La.sub.2 O.sub.3, Yb.sub.2 O.sub.3, Sm.sub.2 O.sub.3, Gd.sub.2 O.sub.3, Nd.sub.2 O.sub.3 and Er.sub.2 O.sub.3, having melting points higher than 2000.degree. C., are expected to be useful for high temperature ceramics.
Japanese Unexamined Patent Publication (Kokai) No. 5-85821 discloses a sintered body comprising a rare earth element oxide (an oxide of a rare earth element or an oxide of two or more rare earth elements) and Al.sub.2 O.sub.3 and a process for producing the same. A rare earth element oxide and Al.sub.2 O.sub.3 are mixed and formed into a shape, followed by sintering the shape at an optimum sintering temperature for an optimum sintering time period so as to control the crystal grain size of the sintered body to 30 .mu.m or less, by which extraordinal grain growth and the appearance of pores are prevented and a rare earth element oxide-alumina sintered body with a high strength, high toughness and high reliability can be provided.
Mr. T. Parthasarathy et al. in Journal of the American Ceramics Society Vol. 76, No. 1, pp29-32 (1993) disclosed a composite of alumina and yttrium aluminum garnet (sometime hereinafter referred to as "YAG") of eutectic Al.sub.2 O.sub.3 --Y.sub.3 Al.sub.5 O.sub.12.
Mr. Parthasarathy et al. also disclose a method of producing the composite by unidirectionally melting and solidifying a mixed powder of Al.sub.2 O.sub.3 and Y.sub.2 O.sub.3 in a crucible.
It is comprehensible from the description on page 29, right column, lines 9 and 10 and FIGS. 1 and 2 of the literature that the composite is polycrystalline and includes grain boundaries. This is clearly supported from the description "The failure was usually along the colony boundaries with crack running along the Al.sub.2 O.sub.3 -YAG interface boundaries". These colony boundaries are shown as portions where the microstructure is larger than in the other portions in FIG. 2 of the literature.
This composite material has stresses equivalent to those of sapphire at 1530.degree. C. and 1650.degree. C. when the strain rate was made constant.
Moreover, the present inventors confirmed by experiment that the composite disclosed by Mr. Parthasarathy et al. include pores or voids in the microstructure and the mechanical strength of the composite falls rapidly at high temperature.
It is clear, as evidenced by the above that the mechanical properties of ceramic composite materials at high temperatures largely depend on the structure of grain boundaries of constituent materials, the interface between the matrix and the reinforcing phase, and crystallographic properties of reinforcing phase and matrix and precise control of these factors are required.
The present inventors, considering the above problems of the conventional art, have vigorously investigated to obtain ceramic composite materials having excellent mechanical strength and creep resistance from room temperature to a high temperature, particularly having remarkably improved properties at high temperatures.
As results, the inventors found novel ceramic composite materials consisting of .alpha.-Al.sub.2 O.sub.3 and YAG, constituting single crystal/single crystal phases, single crystal/polycrystal phases, and polycrystal/polycrystal phases (see Japanese Unexamined Patent Publication (Kokai) Nos. 07-149,597 and 07-187893 and Japanese Patent Application No. 06-240790).